Cellulosic nanocomposite filaments for an ionic strength sensor with ultrahigh precision and sensitivity

Ionic strength sensing plays a crucial role in numerous fields, and there is an urgent demand for portable and robust sensors with rapid and precise detection ability. This study presents a novel micrometer-diameter nanocellulose composite filament sensor, which can detect ionic strength with high accuracy (R2 > 0.998), using only trace amounts of a liquid sample (20 μL). The preparation of such a filament sensor contains two straightforward steps: (1) raw carbon nanotubes (CNTs) are integrated into a spinning dispersion based on cellulose nanofibrils (CNFs) through a facile co-grinding method; (2) composite filaments are obtained through a wet-spinning process to utilize the assembling structures by these two fibrillar nanoparticles, providing superior mechanical properties and baseline conductivity. Such composite filaments confine capillary swelling in nano-channels for controlled ion diffusion, which enables a precise measuring ability for a wide range of ionic strength from 10−5 to 10−1 M within 1 min. The superior selectivity towards different ions, ability to precisely determine ion content for purified water quality, anti-disturbance capacity, and performance stability under varying environmental conditions are examined. Overall, this study paves the way to utilize carbohydrate and carbon materials at nano- and macro-levels through a simple preparation method. In particular, the control of nanoparticles' assembling into macro-sized materials greatly boosts the potential of nanocomposites for practical sensor applications.